A lengthy commentary by the official news agency underlined the North's claim that latest test was of a powerful miniaturised hydrogen bomb which marked a "new high stage" in the country's search for a credible nuclear deterrent.

Experts had largely dismissed the claim, saying the test yield was far too low for a full-fledged thermonuclear device and was similar to the simple fission implosion devices it had tested three times in the past.

News agency's commentary said that the test was an indispensable stage on the "normal course" which other countries had taken to the development of a two-stage fission-fusion H-bomb.